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<p class="MsoNormal" style="margin-left:36.0pt">Notably if a class associated type has a more general kind, we currently can't give a default definition for it that has a tighter kind.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">This is the same situation as holds for default class methods.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">BUT for the latter we invented default method signatures –XdefaultSignatures (user manual
<a href="http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#class-declarations">
Section 9.8.1.4</a>), which can be more restrictive than the signature implied by the method signature of the class.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Maybe we can do the same for default declarations for associated types?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Would someone like to open a ticket and tell the story?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Richard Eisenberg<br>
<b>Sent:</b> 09 March 2016 01:32<br>
<b>To:</b> Edward Kmett <ekmett@gmail.com><br>
<b>Cc:</b> ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: Constrained Type Families?<o:p></o:p></span></p>
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<p class="MsoNormal">I see no good reason for this restriction -- I think that we should just remove the restriction instead of cooking up a workaround. Have you brought this up before? Perhaps make a ticket.<o:p></o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Mar 8, 2016, at 8:24 PM, Edward Kmett <<a href="mailto:ekmett@gmail.com">ekmett@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">If and when that feature lands would it be possible to use it to bypass a current limitation in class associated types?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Notably if a class associated type has a more general kind, we currently can't give a default definition for it that has a tighter kind.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">e.g. I have some classes which are technically polykinded but where 90% of the instances instantiate that kind as *. The status quo prevents me from putting in a type default that would only be valid when the kind argument is *.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-Edward<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Tue, Mar 8, 2016 at 8:21 PM, Richard Eisenberg <<a href="mailto:eir@cis.upenn.edu" target="_blank">eir@cis.upenn.edu</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Mar 8, 2016, at 7:17 PM, Evan Austin <<a href="mailto:e.c.austin@gmail.com" target="_blank">e.c.austin@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">The wiki page for Phase I of Dependent Haskell describes an approach to constrained type families:<o:p></o:p></p>
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<p class="MsoNormal"><a href="https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Phase1#Typefamilyequationscanbeconstrained" target="_blank">https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Phase1#Typefamilyequationscanbeconstrained</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Did that land in GHC 8.0 and, if so, is the updated syntax documented somewhere?<o:p></o:p></p>
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<p class="MsoNormal">No, it didn't make it. The motivating test case seemed contrived and so we punted on this one.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Do you have a use case that really needs this feature? That would help to motivate it for 8.2 or beyond.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks!<o:p></o:p></p>
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<p class="MsoNormal"><span style="color:#888888">Richard<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
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